1. Field of the Invention
The present invention relates to an injection molding machine driving hydraulic actuators by hydraulic pumps, which control discharge flow rates by variably controlling the number of revolutions of a drive motor.
2. Description of the Relevant Art
Conventionally, an injection molding machine that is equipped with a hydraulic pump controlling a discharge flow rate by variably controlling the number of revolutions of a drive motor, and that drives a hydraulic actuator, such as an injecting cylinder (hydraulic cylinder), by the hydraulic pump is known in Japanese Patent Publication No. 3245707.
The objective of the injection molding machine disclosed in the publication is to improve the stability of pressure control by eliminating the fluctuation of the controlled pressure at the time of pressure control, and [the injection molding machine] is equipped with a hydraulic drive source controlling the number of revolutions of a servomotor in fixed discharge type hydraulic pumps. At the same time, a relief circuit that relieves hydraulic oil discharged from the hydraulic pumps is specially arranged in order for the number of revolutions of the hydraulic pumps at the time of pressure control to become the number of revolutions for always being removed from an unstable region of the rotational resistance in the hydraulic pumps or greater.
However, conventional injection molding machine had to overcome the following problems.
First, when the injection molding machine is large in size, since a large servomotor is required in response to the maximum capacity of the injection molding machine, the servomotor itself becomes expensive, increasing the initial cost. Because the upsizing of the servomotor creates a larger capacity (greater electric current) for an auxiliary servo circuit (servo amplifier), ensuring resistance to electric power in the servo circuit and results in the gradual increase of the overall cost.
Secondly, since all operations in the injection molding machine is controlled by one servomotor, a region where the operation capability of the servomotor does not conform to each operational process of the injection molding machine easily occurs. Therefore, control tends to be unstable, which is disadvantageous from the standpoint of securing moldability and molding quality. At the same time, since the overload frequency becomes greater, which is disadvantageous from the standpoint of reliability and longevity, as well. In addition, as a measure against unstable regions where the number of revolutions of the servomotor becomes smaller, when the hydraulic oil to be discharged from the hydraulic pumps is relieved to the oil tank, if one unit of a large-sized servomotor is used, the energy waste becomes great, which is also disadvantageous from the standpoint of energy savings and running costs.